The present invention relates to methods and systems for purifying waste water which include additional sludge treatment by ozonation to reduce significantly the quantities of sludge produced by a biological treatment system, in particular to reduce the cost of treating the sludge resulting from new regulations.
One advantage of said invention is to improve settling of the sludge after treatment.
The invention relates more particularly to a method of purifying waste water charged with organic materials, the method including a step in which the waste water remains in a biological treatment system, referred to as the main biological treatment system (consisting of one or more reactors such as aeration pools, bacteria beds, anaerobic digesters, clarifiers, etc. for biological treatment, possibly in conjunction with physical-chemical treatment), in which said organic materials are degraded by micro-organisms to produce sludge, some of the sludge being subjected to ozonation combined with mechanical stirring before it is sent to the main biological treatment system, the sludge subjected to ozonation in this way being referred to as xe2x80x9ctreated sludgexe2x80x9d.
Document EP-A-0 645 347 describes a method of the above kind in which ozonation takes place after acidifying the treated sludge to a pH less than 5, mechanical stirring being employed either during the acidification process to mix the treated sludge with an acidification reactant or by spraying some of the treated sludge into the ozonation reactor by means of a pump.
The method described in the above document has the drawbacks of requiring relatively large quantities of ozone and interfering with the operation of the main biological treatment system because of the acidification of the treated sludge
One particular object of the present invention is to alleviate these drawbacks.
To this end, the invention proposes a method which is essentially characterized in that during the mechanical stirring step sufficient mechanical energy is imparted to said treated sludge to attack the walls of the micro-organisms contained in the treated sludge.
This improves the efficiency of the ozonation treatment compared to the process described in the document mentioned above because the mechanical stirring energy imparted to the treated sludge is sufficient to weaken the floc and the cell walls of the micro-organisms contained in said treated sludge to enable the ozone to attack the micro-organisms more efficiently. The floc is destroyed by attacking the exopolymers which account for the cohesion of said floc and this causes various bacteria and protozoa to burst.
Quantities of ozone significantly smaller than used in the method described in the document mentioned above can therefore be used.
Moreover, correct operation of the main biological treatment system is not interfered with because it is not necessary to acidify the treated sludge.
Furthermore, the efficiency of the ozonation process is further improved because the treated sludge does not have to be acidified.
Finally, recirculating the sludge to the main biological treatment system absorbs additional pollution generated when the cell walls of the micro-organisms are destroyed (increased chemical oxygen demand [COD], biochemical oxygen demand [BOD] and dissolved organic carbon), the final result being that the volume and mass of the sludge are greatly reduced, the indices of the sludge (in particular the Mohlmann index) are significantly improved and possible biological disorders (in particular xe2x80x9cbulkingxe2x80x9d due to filamentary bacteria) are minimized.
One or more of the following features can be used in preferred embodiments of the invention:
the pH of the treated sludge is always from 6 to 9;
the treated sludge is mechanically stirred before its ozonation;
the treated sludge is mechanically stirred after its ozonation;
the mechanical stirring and the ozonation of the treated sludge take place in the same reaction enclosure;
a particular flowrate of treated sludge, referred to as the first flowrate, is taken from the outlet of a treated sludge ozonation reactor, this first flowrate then being subjected to mechanical stirring, said first flowrate being then sent to the ozonation reactor with a particular additional flowrate of sludge from the main biological reactor, referred to as the second flowrate, the second flowrate being lower than the first flowrate;
the treated sludge is subjected to aerobic or anaerobic digestion in addition to ozonation and mechanical stirring;
the aerobic or anaerobic digestion takes place after ozonation and mechanical stirring;
a particular flowrate of the treated sludge is taken from the outlet of a digester in which the treated sludge undergoes the aerobic or anaerobic digestion, this flowrate of treated sludge then being subjected to mechanical stirring and ozonation before it is sent to the digester with a particular additional flowrate of sludge from the main biological reactor;
the main biological treatment system is sent only some of the treated sludge that has been subjected to aerobic or anaerobic digestion and further treated sludge leaving the digester is evacuated;
the ozonation step is implemented in an ozonation reactor which includes at least one vent from which exits a gaseous effluent including at least ozone and oxygen, the method further including a step of collecting this gaseous effluent and re-using said gaseous effluent to treat the waste water or other liquid resulting from the treatment of the waste water;
the ozone contained in the gaseous effluent collected from the outlet of the vent is destroyed thermally or catalytically before said gaseous effluent is re-used;
the mechanical stirring energy is from 10 kJ to 20,000 kJ per kg of dry material of the treated sludge;
the mechanical stirring energy is from 50 kJ to 3,000 kJ per kg of dry material of the treated sludge.
The invention also provides a system for implementing a method as defined hereinabove, the system including a pressurized ozonation reactor in which the treated sludge is subjected to ozonation.
Other features and advantages of the invention will become apparent in the course of the following description of several embodiments of the invention given by way of non-limiting example and with reference to the accompanying drawings.